The problem of separating different polymeric materials from each other is the primary obstacle to economically recycling polymeric materials from durable goods. Three main separation techniques currently exist: hand sorting, optical/spectroscopic based sorting, and mechanical sorting based primarily on differences in material density. Hand sorting is labor intensive and cost-prohibitive in most cases. Optical-based sorting techniques, which include visible, near-infrared, ultraviolet, and X-ray fluorescence, have many applications, and these systems will certainly play a role in future recycling plants as described, for example, in commonly assigned International Application No. PCT/US03/11642, filed Apr. 14, 2003, which is incorporated herein by reference. However, no system available today is broadly applicable to polymeric materials containing fillers, paints, coatings, and other contaminants. Mechanical sorting systems take advantage of physical property differences between materials to segregate them. The most common physical property that these techniques exploit is a difference in material density.
Density sorting is used to upgrade a variety of raw materials including mixed plastics, coal and mixed metals. It is frequently necessary to perform these separations in a fluid with a density greater than that of water. The density of a fluid can be increased by adding a solid material (which will be referred to as “media”,) to the fluid to create a slurry. Magnetite, titanium dioxide, sand, ferrosilicate or other materials are frequently added to water to adjust the density upwards so that material with a density as high as 2.6 g/cc or higher can be made to float. Separations making use of such slurried media will be referred to as “slurried dense media separations”.
A hydrocyclone or other density separation device can be used to separate materials by density. Some density separation devices introduce liquid and particles that are to be separated (mixture particles) into a conical or cylindrical device. A vortex is created within the device causing the particles that are denser than the liquid to report to the bottom of the device (the underflow) and particles that are less dense than the liquid to report to the top of the device (the overflow). When using a separation fluid that is a slurry of dense mineral materials in water, the slurry can be unstable and subject to settling. The media particles used to increase the density of the fluid should be small enough that they will not settle as rapidly as the particles of the mixture to be separated. For example, if magnetite is used as a media, the particle size must typically be smaller than 200 microns to ensure that the magnetite does not settle so rapidly that the fluid does not behave as a dense fluid. A number of density separation devices are described in commonly-assigned U.S. Pat. No. 6,238,579, which is incorporated by reference herein.
In addition to separating mixtures of particles, hydrocyclones can also be used to dewater solids and classify solids by size. FIG. 1 illustrates the configuration of a typical hydrocyclone 100. The particle size of solid that a hydrocyclone is capable of recovering from a slurry is determined by a combination of variables, including the cyclone diameter 110, the inlet area 120, the diameter of the vortex finder 130, the height of the classification section 140, the flow rate of feed into the cyclone, the density of the solid particles in the slurry and the density of the separation fluid.